In general, electronic appliances include a key input circuit for receiving an operation signal from the exterior. Such key input circuit includes a key input unit that generates a key input signal from the exterior and a microcomputer that controls an operation of an electronic appliance upon recognizing the key input signal.
One key input method of the related art key input circuit is to recognize an operation signal of a user based on a difference between divided voltages generated according to a key input by using DC power and resistors.
In this method, however, because the key input unit and the microcomputer use a common ground, if a ground voltage level itself of the microcomputer has a certain potential, there is a possibility of the danger of an electrical shock.
Meanwhile, another key input method of the related art key input circuit is to completely separate the key input unit and power unit connected with the microcomputer by using a transformer.
That is, unlike the voltage dividing method, a key connected with a secondary side of the transformer is pressed to change voltage at the secondary side, and such change causes a change in voltage at a primary side of the transformer. At this time, a peak value of the generated voltage of the primary side is detected in order to input an operation signal of the user to the microcomputer.
In this method, the key input unit and the power unit are completely separated, so there is no danger of an electrical shock. In this sense, the transformer is called an insulation transformer.
However, the key input circuit according to this method has a problem in that because a signal transmission ratio, namely, a ratio of the primary side voltage to the secondary side voltage, is not good due to an operation deviation of the insulation transformer, the key input circuit may perform a different operation that does not correspond to a key input signal.